Good evening,
In theory this could be any time between1902 when this uniform was first issued and 1938 when it was replaced by the universal battledress.
In practice it is likely to be circa WW1 when most men had there mug shots done in uniform for the family. If it's actually during the war then not between 1916 1918 when the breast pockets had no pleats.
He is royal artillery but I can't tell you more than that.
John915